forge_fdf checkbox exception

The following works fine if I only fill in the fields of the PDF (i.e. the boolean_a field is an empty list). But if I add entries to the list for checkboxes, I get the exception given below.:

field_a = [[b'Date Prepared', 'October 30, 2016']]
boolean_a = [[b'Check Box1', 'Yes']]

form = 'form.pdf'
fdf = 'temp.fdf'
output = 'output.pdf'
write_sect(field_a, boolean_a, fdf, form, output)


def write_sect(fields, booleans, fdf, form, output):

    forged_fdf = forge_fdf('', fields, booleans, [], [])

    with open(fdf, 'wb') as tmp_fw:
        tmp_fw.write(forged_fdf)

    call('pdftk "{0}" fill_form "{1}" output "{2}" dont_ask'.format(form, fdf, output))

    return

I get the following error:

Unhandled Java Exception in create_output():
java.lang.ClassCastException: pdftk.com.lowagie.text.pdf.PdfNull cannot be cast to pdftk.com.lowagie.text.pdf.PdfDictionary
   at 0x0059a84e (Unknown Source)
   at 0x0059ad42 (Unknown Source)
   at 0x005e9bd4 (Unknown Source)
   at 0x005ba4a4 (Unknown Source)
   at 0x005b2044 (Unknown Source)
   at 0x0059231e (Unknown Source)
   at 0x004723f1 (Unknown Source)
   at 0x00472045 (Unknown Source)
   at 0x004df3e2 (Unknown Source)
   at 0x004df38a (Unknown Source)
   at 0x00471e74 (Unknown Source)